The invention relates to an on-board-diagnosis method for an exhaust aftertreatment system and an on-board-diagnosis system for an exhaust aftertreatment system.
Present regulatory conditions in the automotive market have led to an increasing demand to improve fuel economy and reduce emissions in present vehicles. These regulatory conditions must be balanced with the demands of a consumer for high performance and quick response for a vehicle.
A diesel engine has an efficiency of up to about 52% and is thus the best converter of fossil energy. NOx emission concentration, i.e. the emission of nitrogen oxides NO and NO2, is dependent upon local oxygen atom concentration and the local temperature. Said high efficiency is however only possible at an elevated combustion temperature at which high NOx levels are inevitable. Moreover, a suppression of NOx formation by internal means (air/fuel ratio) has the tendency to cause an increase in particulates, known as the NOx—particulates trade off. Furthermore, an excess of oxygen in the exhaust gas from a diesel engine prevents the use of stoichiometric 3-way-catalyst technology for reduction of NOx as, is used in gasoline engine oars from the late 80-ties.
Both carbon particulates and NOx are typical emissions in the exhaust gas of diesel engines. Requirements for reducing such emissions increase and trigger various approaches in the art to reduce emissions. In the European patent EP 1 054 722 B1 an exhaust aftertreatment system is disclosed which combines a particulate filter collecting soot and nitrogen-oxides reduction catalysts in the exhaust tract. For removing soot NO2 is generated by oxidation of NO in an oxidation catalyst. Soot which is collected in a particulate filter is oxidized by NO2. Residual amounts of NO and NO2 in the exhaust gas are reduced to nitrogen gas in a selective-catalytic-reduction catalyst (SCR catalyst) by injecting ammonia into the SCR catalyst.
During operation all catalysts degrade due to accumulation of poisons, thermal migration of the catalyst material etc. This degradation seriously influences the operation of aftertreatment systems. Therefore it is desirable to detect the degradation of a catalyst in the aftertreatment system before the operation of the aftertreatment system fails or legal requirements cannot be fulfilled because of the degradation. This is done by the so called OBD (On-Board Diagnosis).
It is desirable to provide an improved on-board-diagnosis method for an exhaust aftertreatment system. It is also desirable to provide an adequate improved exhaust aftertreatment system.
An on-board-diagnosis method for an exhaust aftertreatment system of an engine is proposed comprising a diesel particulate filter unit which encompasses a particulate filter (DPF) in which soot can be oxidized by NO2 and constituents of the exhaust gas are deoxidized in a selective-catalytic-reduction (SCR) catalyst arranged downstream of the diesel particulate filter unit, wherein the exhaust gas flows from the diesel particulate filter unit to the selective-catalytic-reduction catalyst. According to invention an NO2 content upstream of the nitrogen-oxides reduction unit is estimated; at least one of an NOx or NO2 content upstream of the nitrogen-oxides reduction unit is measured; a comparison between the estimated and the measured contents is performed; and based on the result of the comparison it is decided on at least one conditioning step of the diesel particulate filter unit.
Favourably, the proper operation of the diesel particulate filter unit can be reliably checked on board of a vehicle which is equipped with said aftertreatment system, improving the environmental compatibility of diesel engines. This can be done based on the NO2 level in the exhaust gas on particular locations and/or based on the efficiency of the nitrogen-oxides reduction unit.
Preferably for high load operation conditions of the engine, the following steps can be performed: measuring the NO2 content in the exhaust gas upstream of the nitrogen-oxides reduction unit and downstream of the diesel particulate filter unit; calculating the NO2 content in the exhaust gas upstream of the nitrogen-oxides reduction unit and downstream of the diesel particulate filter unit; measuring an NOx content in the exhaust gas downstream of the nitrogen-oxides reduction unit; measuring or calculating an NOx content in the exhaust gas upstream of the nitrogen-oxides reduction unit and downstream of the diesel particulate filter unit; comparing the measured and the expected NO2 contents; and based on the result of the comparison deciding on at least one conditioning step of the diesel particulate filter unit.
Favourably the operability of the diesel particulate filter unit can be determined with high accuracy.
Preferably for low load operation conditions of the engine, the following steps can be performed: calculating the NO2 content in the exhaust gas upstream of the nitrogen-oxides reduction unit and downstream of the diesel particulate filter unit; measuring or measuring and calculating an NOx content in the exhaust gas upstream of the nitrogen-oxides reduction unit and downstream of the diesel particulate filter unit; measuring the NOx content in the exhaust gas downstream of the nitrogen-oxides reduction unit; and deriving the efficiency of the NOx conversion in the nitrogen-oxides reduction unit from the calculated NO2 and the measured and calculated NOx contents. Favourably, a real NO2 sensor is not necessary as the NO2 content can be calculated, this embodiment thus needs less hardware and is cost efficient. If the efficiency of the nitrogen-oxides reduction unit is not sufficient, a separate diagnosis for the nitrogen-oxides reduction unit as well as for a reducing-agent system (in case a SCR catalyst is used with injection of a reducing agent), can be triggered. This can be done after the regeneration related to the diesel particulate filter unit or before the regeneration.
Preferably, a conditioning step can be performed if the difference between the compared NOx conversions and/or the compared NO2 contents are beyond a predetermined value.
Particularly, the NOx conversion or the compared NO2 contents in the exhaust gas can be derived by determining the NO2 content with a real sensor and/or a virtual sensor.
Deriving the NOx conversion in the exhaust gas can be advantageously done by determining an efficiency of the nitrogen-oxides reduction unit.
Another advantage with the present invention is that one may achieve high passive regeneration and HC oxidation in the DPF system and maintaining a good NO2/NO ratio for high NOx-conversion in the SCR-system for a fresh as well as an aged system. Advantageously, the preferred arrangement allows to using a smaller SCR-catalyst, giving both cost, space and weight benefits. The diesel particulate filter unit (DPFU) may have the oxidation catalyst (DOC) upstream of the diesel particulate filter (DPF). Alternatively or additionally, the DPF may comprise a catalytic coating which oxidizes exhaust gas components and which can replace or support the DOC. An advantage with this embodiment is that one will still further save space, cost and weight.
Preferably, a conditioning step can be performed if the differences between the compared NOx conversions and/or the compared NO2 contents are beyond a predetermined value.
The NOx conversion and/or the NO2 contents can be derived by determining the NO2 content in the exhaust gas with a real sensor and/or a virtual sensor. Alternatively or additionally, NOx conversion can be derived from the efficiency of the nitrogen-oxides reduction unit without using a real NO2 sensor.
Preferably a first conditioning step can be performed by heating an oxidation stage in the diesel particulate filter unit to at least 350° C., preferable to a temperature between 350° C. and 450 C.
Favourably, a second conditioning step can be performed by heating an oxidation stage in the diesel particulate filter unit to at least 450° C., preferable to a temperature between 450° C. and 550° C.
Advantageously, a third conditioning step is performed by heating an oxidation stage in the diesel particulate filter unit to at least 550° C., preferable to a temperature between 550° C. and 650° C.
Particularly, the second conditioning step can be performed after the first conditioning step and the third conditioning step can be performed after the second conditioning step. After each conditioning step it is decided if a further conditioning step has to be performed or not. The thermal load to the particulate filter unit is favourably reduced to the bare necessary heating steps and temperatures.
Wall conditioning steps have been unsuccessful, after performing all conditioning steps unsuccessfully, an alarm can be set.
According to another aspect of the invention, an exhaust aftertreatment system of an engine is proposed comprising a diesel particulate filter unit which encompasses a particulate filter in which soot can be oxidized by NO2 and constituents of the exhaust gas are deoxidized in a nitrogen-oxides reduction unit arranged downstream of the diesel particulate filter unit, wherein the exhaust gas flows from the diesel particulate filter unit to the nitrogen-oxides reduction unit, wherein the operability of at least the diesel particulate filter unit can be determined by calculating the NO2 content in the exhaust gas upstream of the nitrogen-oxides reduction unit and downstream of the diesel particulate filter unit; measuring or calculating an NOx content in the exhaust gas upstream of the nitrogen-oxides reduction unit and downstream of the diesel particulate filter unit; measuring the NOx content in the exhaust gas downstream of the nitrogen-oxides reduction unit; calculating the NOx content in the exhaust gas upstream of the nitrogen-oxides reduction unit and downstream of the diesel particulate filter unit; and deriving the actual and the expected NOx conversion from the calculated NO2 and the measured and calculated NOx contents.
Preferably an NO2 sensor can be provided upstream of the selective-catalytic-reduction catalyst and downstream of the diesel particulate filter unit. Additionally or alternatively, the NO2 sensor can be provided downstream of the nitrogen-oxides reduction unit. The NO2 sensor can be a real sensor implemented as hardware or a virtual sensor implemented as software where the NO2 content is calculated based on appropriate operation parameters of the engine and the exhaust gas aftertreatment system.
According to a further aspect of the invention, a computer program is proposed which is storable on a computer readable medium, comprising a program code for use in an on-board-diagnosis method for an exhaust aftertreatment system of an engine comprising a diesel particulate filter unit which encompasses a particulate filter in which soot can be oxidized by NO2 and constituents of the exhaust gas are deoxidized in a nitrogen-oxides reduction unit arranged downstream of the diesel particulate filter unit, wherein the exhaust gas flows from the diesel particulate filter unit to the selective-catalytic-reduction catalyst, comprising at least the steps of calculating the NO2 content in the exhaust gas upstream of the nitrogen-oxides reduction unit and downstream of the diesel particulate filter unit; measuring or calculating an NOx content in the exhaust gas upstream of the nitrogen-oxides reduction unit and downstream of the diesel particulate filter unit; measuring the NOx content in the exhaust gas downstream of the nitrogen-oxides reduction unit; calculating the NOx content in the exhaust gas upstream of the nitrogen-oxides reduction unit and downstream of the diesel particulate filter unit; and deriving the actual and the expected NOx conversion from the calculated NO2 and the measured and calculated NOx contents.
In the drawings, equal or similar elements are referred to by equal reference numerals. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. Moreover, the drawings are intended to depict only typical embodiments of the invention and therefore should not be considered as limiting the scope of the invention.